The link to discussion, provided by Catchdave, has another valid point (original, dead link) made by Dick Hardt, which I believe is worth to be mentioned here in addition to what’s been written above:
My recollection of refresh tokens was for security and revocation.
<…>revocation: if the access token is self contained, authorization can be revoked by not issuing new access tokens. A resource does not need to query the authorization server to see if the access token is valid.This simplifies access token validation and makes it easier to scale and support multiple authorization servers. There is a window of time when an access token is valid, but authorization is revoked.
Indeed, in the situation where Resource Server and Authorization Server is the same entity, and where the connection between user and either of them is (usually) equally secure, there is not much sense to keep refresh token separate from the access token.
Although, as mentioned in the quote, another role of refresh tokens is to ensure the access token can be revoked at any time by the User (via the web-interface in their profiles, for example) while keeping the system scalable at the same time.
Generally, tokens can either be random identifiers pointing to the specific record in the Server’s database, or they can contain all information in themselves (certainly, this information have to be signed, with MAC, for example).
How the system with long-lived access tokens should work
The server allows the Client to get access to User’s data within a pre-defined set of scopes by issuing a token. As we want to keep the token revocable, we must store in the database the token along with the flag “revoked” being set or unset (otherwise, how would you do that with self-contained token?) Database can contain as much as len(users) x len(registered clients) x len(scopes combination) records. Every API request then must hit the database. Although it’s quite trivial to make queries to such database performing O(1), the single point of failure itself can have negative impact on the scalability and performance of the system.
How the system with long-lived refresh token and short-lived access token should work
Here we issue two keys: random refresh token with the corresponding record in the database, and signed self-contained access token, containing among others the expiration timestamp field.
As the access token is self-contained, we don’t have to hit the database at all to check its validity. All we have to do is to decode the token and to validate the signature and the timestamp.
Nonetheless, we still have to keep the database of refresh tokens, but the number of requests to this database is generally defined by the lifespan of the access token (the longer the lifespan, the lower the access rate).
In order to revoke the access of Client from a particular User, we should mark the corresponding refresh token as “revoked” (or remove it completely) and stop issuing new access tokens. It’s obvious though that there is a window during which the refresh token has been revoked, but its access token may still be valid.
Tradeoffs
Refresh tokens partially eliminate the SPoF (Single Point of Failure) of Access Token database, yet they have some obvious drawbacks.
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The “window”. A timeframe between events “user revokes the access” and “access is guaranteed to be revoked”.
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The complication of the Client logic.
without refresh token
- send API request with access token
- if access token is invalid, fail and ask user to re-authenticate
with refresh token
- send API request with access token
- If access token is invalid, try to update it using refresh token
- if refresh request passes, update the access token and re-send the initial API request
- If refresh request fails, ask user to re-authenticate
I hope this answer does make sense and helps somebody to make more thoughtful decision. I’d like to note also that some well-known OAuth2 providers, including github and foursquare adopt protocol without refresh tokens, and seem happy with that.